Method for forming a compound semi-conductor thin-film

ABSTRACT

A method is provided for fabricating a thin film semiconductor device. The method includes providing a plurality of raw semiconductor materials. The raw semiconductor materials undergo a pre-reacting process to form a homogeneous compound semiconductor target material. The compound semiconductor target material is deposited onto a substrate to form a thin film having a composition substantially the same as a composition of the compound semiconductor target material.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. Ser. No. 12/896,120,filed Oct. 1, 2010, which is a divisional of U.S. Ser. No. 12/061,450,filed Apr. 2, 2008, now U.S. Pat. No. 7,842,534, both of which areincorporated herein by reference in their entireties.

FIELD OF INVENTION

The present invention relates to a method for forming a compoundsemiconductor thin-film such as a semiconductor thin-film suitable foruse in photovoltaic solar cells and other devices.

BACKGROUND OF THE INVENTION

Photovoltaic devices represent one of the major sources ofenvironmentally clean and renewable energy. They are frequently used toconvert optical energy into electrical energy. Typically, a photovoltaicdevice is made of one semiconducting material with p-doped and n-dopedregions. The conversion efficiency of solar power into electricity ofthis device is limited to a maximum of about 37%, since photon energy inexcess of the semiconductor's bandgap is wasted as heat. Thecommercialization of photovoltaic devices depends on technologicaladvances that lead to higher efficiencies, lower cost, and stability ofsuch devices. The cost of electricity can be significantly reduced byusing solar modules constructed from inexpensive thin-filmsemiconductors such as copper indium di-selenide (CuInSe₂ or CIS) orcadmium telluride (CdTe). Both materials have shown great promise, butcertain difficulties have to be overcome before their commercialization.

As shown in FIG. 1, the basic form of a CIS, or CdTe, compoundsemiconductor thin-film solar cell (1) comprises of a multilayerstructure superposed on a substrate (2) in the following order, a backelectrode (3), a light absorbing layer (4), an interfacial buffer layer(5), a window layer (6) and an upper electrode (7). The substrate iscommonly soda-lime glass, metal ribbon or polyimide sheet. The backelectrode is commonly a Mo metal film.

The light absorbing layer consists of a thin-film of a CIS p-typeCu-III-VI₂ Group chalcopyrite compound semiconductor. e.g copper indiumdi-selenide (CIS). Partial substitutions of Ga for In (CIGS) and/or Sfor Se (CIGSS) are common practices used to adjust the bandgap of theabsorber material for improved matching to the illumination.

CIS and similar light absorbing layers are commonly formed by variousprocessing methods. These include Physical Vapor Deposition (PVD)processing in which films of the constituent elements are simultaneouslyor sequentially transferred from a source and deposited onto asubstrate. Standard PVD practices and equipment are in use across manyindustries.

PVD methods include thermal evaporation or sublimation from heatedsources. These are appropriate for elemental materials or compoundswhich readily vaporize as molecular entities. They are less appropriatefor multi-component materials of the type discussed here, which maydecompose and exhibit preferential transport of subcomponents.

A sub-set of PVD methods are appropriate for single-source,multi-component material deposition, including magnetron sputtering andlaser ablation. In these implementations multi-component compounds orphysical mixtures of elements, or sub-compounds, are formed intotargets. The targets are typically unheated or cooled, and their surfaceis bombarded with high energy particles, ions or photons with theobjective that the surface layer of the target is transported incompositional entirety. In this way complex materials can be deposited.By this means the target composition and molecular structure can beclosely replicated in the film. Exceptions occur, most commonly, whentargets are made from physical mixtures of elemental or sub-componentsrather than fully reacted compounds. In such cases the target may beconsumed non-uniformly with preferential transport of constituents.

Targets for PVD may be formed by casting, machining or otherwisereforming bulk materials. This includes pressing of powdered materials.Target shapes and sizes vary in different systems and at the end of lifethe spent target materials are frequently recycled.

In the case of CIS and similar light absorbing layers, evaporation isoften used to deposit films of the constituent elements onto thesubstrate. This may be carried out under a reactive atmosphere of theChalcogen (Se or Se) or, alternatively, these elements can besubsequently introduced by post processing in reactive atmospheres (e.g,H₂Se). Heating of the substrates during deposition and/orpost-deposition processing, at temperatures in the range of 500° C. forextended periods, is often employed to promote mixing and reaction ofthe film components in-situ.

A shortcoming of the conventional multi-source vacuum deposition methodsis the difficulty in achieving compositional and structural homogeneity,both in profile and over large areas for device manufacturing. Morespecifically, device performance may be adversely impacted as a resultof such inhomogeneities, including semi-conducting properties,conversion efficiencies, reliability and manufacturing yields. Attemptsto remove inhomogeneity through post-deposition processing are imperfectand can generate other detrimental effects. Such post-processing istypically carried out at sub-liquidus temperatures of the thin-filmmaterial.

When simultaneous deposition from confocal sources for the constituentsis employed, the film composition differs from the designed compositionoutside the focal region. Such methods are suitable for demonstration,but are more limited in achieving uniform large area depositions as isenvisioned for low-cost device manufacturing.

When the deposition is from sequential, or partially overlapping,sources (e.g, strip effusion cells used for in-line processing), theelemental composition of the films vary in profile. Subsequent mixingthrough thermal inter-diffusion is typically imperfect especially if theprocessing is constrained by the selection of substrates and other cellcomponents. As an example, in-line deposited GIGS films exhibit gradedGa and In concentrations, consistent with the sequence and overlap ofthe elemental sources.

In Photovoltaic and other multi-layer devices, reactions and diffusionat film interfaces during deposition or during post-depositionprocessing may impact performance. For instance it has been shown thatNa thermally diffuses from soda-lime substrates into CIS layers. In thisinstance the effect is found considered beneficial to the performance ofthe photovoltaic cell. Such effects are a natural consequence of thestandard processing and are not independently controlled.

CIGS solar cells have been fabricated at the National Renewable EnergyLaboratory (NREL) in Colorado which demonstrate 19.5% conversionefficiency under AM 1.5 illumination. These are small area devicesproduced by elemental co-evaporation onto soda-lime substrates. Largerarea devices, manufactured by vacuum and non-vacuum methods by variousentities, on glass, metal ribbon or polymer substrates, more typicallydemonstrate conversion area efficiencies in the range of 8-12%. It isgenerally accepted that it is due to shortcomings in the deviceprocessing, including the light absorbing layer. In this layer, therecan be incomplete compositional non-uniformity, incomplete chemicaland/or structural development and/or other defects across area, profileand at the interfaces of the layer.

The aforementioned techniques for manufacturing CIS-based semiconductorthin-films for use in photovoltaic devices have not resulted in costeffective solutions with conversion efficiencies that are sufficient formany practical applications.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided forfabricating a thin-film semiconductor device. The method includesproviding a plurality of raw semiconductor materials. The rawsemiconductor materials undergo a pre-reacting process to form ahomogeneous compound semiconductor target material. This pre-reactiontypically includes processing above the liquidus temperature of thecompound semiconductor. The compound semiconductor target material isdeposited onto a substrate to form a thin-film having a compositionsubstantially the same as a composition of the compound semiconductortarget material.

In accordance with one aspect of the invention, the raw semiconductormaterials may include II-VI compound semiconductor materials.

In accordance with another aspect of the invention, the II-VI compoundsemiconductor materials may be selected from the group consisting ofCd—S, Cd—Se, Cd—Te, Cd—Zn—Te, Cd—Hg—Te, and Cu—In—Se.

In accordance with another aspect of the invention, Cu may be providedas a minor constituent along with the II-VI compound semiconductormaterials.

In accordance with another aspect of the invention, the rawsemiconductor materials may include I-III-VI compound semiconductormaterials.

In accordance with another aspect of the invention, the I-III-VIcompound semiconductor materials may be selected from the groupconsisting of Cu—In—Se, Cu—In—Ga—Se, Cu—In—Ga—Se—S.

In accordance with another aspect of the invention, Al may be providedin complete or partial substitution for Ga.

In accordance with another aspect of the invention, Na may be providedas a minor constituent along with the I-III-VI compound semiconductormaterials.

In accordance with another aspect of the invention, an alkali elementother than Na may be provided as a minor constituent long with theI-III-VI compound semiconductor materials.

In accordance with another aspect of the invention F may be provided asa minor constituent along with the I-III-VI compound semiconductormaterials.

In accordance with another aspect of the invention, a halogen elementother than F may be provided as a minor constituent along with theI-III-VI compound semiconductor materials.

In accordance with another aspect of the invention, the compoundsemiconductor target material may be deposited by physical vapordeposition.

In accordance with another aspect of the invention, the compoundsemiconductor target material may be deposited by magnetron sputtering.

In accordance with another aspect of the invention, the compoundsemiconductor target material may be deposited by laser ablation.

In accordance with another aspect of the invention, pre-reacting the rawsemiconductor materials may include isolating the raw semiconductormaterials at an elevated temperature to establish a structure andbonding profile representative of the deposited thin-film.

In accordance with another aspect of the invention, a photovoltaicdevice, is provided which includes a substrate, a first electrodedisposed on the substrate, and a light absorbing layer that includes aReacted Target Physical Deposition (RTPD) compound semiconductorthin-film. A second electrode is disposed over the light absorbinglayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a CIS or CdTe compoundsemiconductor thin-film solar cell.

FIG. 2 is a flowchart showing one example of a process that may beemployed to fabricate a thin-film semiconductor device such as aphotovoltaic device.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of exemplaryembodiments or other examples described herein. However, it will beunderstood that these embodiments and examples may be practiced withoutthe specific details. In other instances, well-known methods,procedures, components and circuits have not been described in detail,so as not to obscure the following description. Further, the embodimentsdisclosed are for exemplary purposes only and other embodiments may beemployed in lieu of, or in combination with, the embodiments disclosed.

The processes of the present invention can be used to fabricatehigh-quality thin-film copper indium di-selenide (CIS)-basedsemiconductor devices that have photovoltaic effects and are especiallyadaptable for solar cell applications. For purposes of simplicity, thedescription of the processes of this invention will focus primarily onCIS-based structures. However, it should be understood that theprocesses and techniques described herein are also applicable to thefabrication of a wide variety of other compound semiconductor thin-filmstructures.

The processes of the present invention involve a Reacted Target PhysicalDeposition (RTPD) method, which provides an alternative path for formingcompound semiconductor thin-films. The RTPD method can produce a uniformfilm with a designated composition while requiring reduced levels ofpost-deposition processing in comparison to other establishedfabrication methods. These attributes impart multiple advantages for thedesign, performance and manufacture of such films for photovoltaic solarcells and other devices.

Thin-films fabricated in accordance with the RTPD method will haveimproved compositional, chemical and structural uniformity in bothprofile and in area. As a consequence, properties which are dependent onuniformity can be improved. Thermal and/or chemical post-depositionprocesses that are conventionally used to approach such uniformity canalso be reduced or eliminated. The latter will be beneficial forminimizing interactions between device layers, process simplificationand manufacturing cost reduction.

The RTPD process is comprised of two parts. Specifically, (i) theproduction of a “pre-reacted” target of designed composition, which iscompositionally uniform and has a molecular structure which isrepresentative of the fully formed compound semiconductor and (ii) theuse of a physical deposition method, such as magnetron sputtering, whichis well suited for depositing thin-film with identical, or nearidentical, composition and molecular structure to the identified target.

For the RTPD method the pre-reacted target may be made by any availablemethod of synthesis. The key attribute of the target is its pre-reactednature, which provides a source material for deposition which has adefined composition and molecular structure. This pre-reaction typicallyincludes processing above the liquidus temperature of the compoundsemiconductor. The objective of the pre-reaction process is to achievethe homogenization and structural definition attributes in the target,in order to simplify film deposition and post processing.

In contrast to the RTPD process, conventional targets made from simplephysical mixtures of un-reacted elements or sub-components are generallyunsatisfactory for many purposes. The reasons being that (i) thestructure and bonding of the compound is not pre-established and (ii)segregated components are likely to exhibit preferential transport.

In some examples, the RTPD process is used to fabricate thin-filmcompound semi-conductors having layer thicknesses less than about 10micrometers thick, and more specifically having layer thicknesses of afew micrometers. In regard to CIS and similar materials of the type usedin solar cells, the thin-films are typically polycrystalline ormicrocrystalline in morphology. This is distinct from the structure ofepitaxially grown single crystal films, which are also sometimes used incertain solar cell devices. This is also distinct from the structure ofsome amorphous films, such as amorphous Si, which are sometimes used inother solar cell devices. The RTPD method employs deposition processes,as noted in the above examples, which can produce amorphous orpolycrystalline films. Their form can be influenced by the temperatureof the substrate during deposition or by thermal annealing afterdeposition.

For compound semiconductors the preparation of bulk materials and RTPDtargets typically require a special approach. Specifically, the rawmaterials (“the batch”), which include volatile elements such as theChalcogens, S, Se or Te, should be contained in their entirety in acrucible made from a material which will not react significantly withthe target materials or the processing atmosphere. For example, carboncrucibles or carbon coated silica glass vessels may be used. Silicavessels may be evacuated and sealed around the batch of raw materials.Alternatively, crucibles may be placed in a sealed chamber under anappropriate pressure of inert gas such as Argon.

Sealed vessels may be processed in conventional ovens or furnaces. Inpressurized chambers heating is provided locally to the crucible and thebatch while the chamber walls are unheated or cooled.

The batch should be processed at a sufficient temperature, typicallyabove the liquidus temperature of the compound semiconductor, for asufficient time to allow the constituents to mix and chemically react sothat they may form a homogeneous compound semiconductor body. Enablingsuch mixing and chemical reaction to occur in the batch will minimizethe post-processing required for the RTPD thin-films. Typical processingcan be at temperatures in the region of 1000° C. under pressures of manyatmospheres.

Certain aspects of the Czochralski single crystal growth method (see,for example, U.S. Pat. No. 4,652,332 by Ciszek) are similar to RTPDtarget synthesis as they relate to batch processing in a sealed chamber.The goal of the Czochralski method is to pull a single crystal from amelt. In the RTPD method, the goal is wider, covering differentprocessing, solidification practices and material objectives.

During processing in evacuated vessels, equilibrium vapors are formedabove the batch. During processing in pressurized chambers, vaporizationis inhibited by the overpressure of the inert gas. Component loss in theformer case is compensated by adjusting the batch formulation.

The compound semiconductor target produced from the batch in theaforementioned manner may be amorphous, polycrystalline or singlecrystal in form. This form may be influenced as desired by adjusting,for example, the cooling rate of the batch after high temperatureprocessing.

The RTPD approach also enables beneficial dopants, such as Na in thecase of CIS solar cells, to be introduced more uniformly in depositedthin-films by incorporating appropriate precursors in the batch.

The target may be used in the shape in which it is formed in the vesselor crucible. Optionally, the target may be machined or otherwisereformed to an appropriate shape for a given deposition system. In thecase of magnetron sputtering targets typical shapes include circular orrectangular plates which are bonded to a metal carrier to facilitatebackside cooling during the deposition process.

The compound semiconductor thin-films deposited by the RTPD method usingtargets fabricated in the manner described above are characterized inthat they are uniform in composition and have substantially developedmolecular structure and bonding in their as-deposited form, as requiredin final product. In such thin-films, the optimization of their grainsizes and other desirable features requires less additional processingin terms of time at temperature than films formed by conventionaltechniques. When conventional techniques are employed, thermalprocessing is also required to produce the mixing and chemical reactionof the components, whereas in RTPD thin-films these issues are addressedin advance.

The RTPD method can impart many advantages for compound semiconductorthin-film processing over current methods. These include:

-   1. Processing the batch materials at higher temperatures than in    conventional approaches. This facilitates better mixing and more    complete reaction of the constituents.-   2. Improved compositional control of the principal constituents by    target composition. This can facilitate bandgap engineering without    processing changes, e.g, the relationship between composition and    the bandgap of the compound semiconductor    CuIn_((1-x))Ga_(x)(S_(y)Se_((1-y)))₂ is given by:

E _(g)=0.95+0.8x−0.17×(1−x)+0.7y−0.05y(1−y)  (1)

-   3. Improved doping of the thin-films through pre-doping of targets    with, e.g, Na.-   4. More uniform films with fewer vacancies or other performance    limiting defects.-   5. Thinner films for reduced materials usage for cost reduction.-   6. Films with reproducible compositions over larger areas. For    higher yield, throughput and low-cost manufacturing.-   7. Process simplifications including (i) elimination of multi source    deposition, monitoring and control and (ii) elimination of chemical    (e.g Selenization) and/or thermal post-processing steps.-   8. If desired, multiple RTPD processing can be used simultaneously    or in sequence to engineer compositional grading. It may also be    used in combination with other deposition techniques.

By the RTPD method, CIS or similar films can be produced which are moreuniform in composition over area and in profile than can achieved bycurrent practices. This uniformity is expected to impart a higherperformance, reproducibility and manufacturability for CIS solar cells.

EXAMPLES

-   1. In one embodiment, a target for RTPD processing is formed from a    compound semiconductor material having a specified composition.    Examples of which are presented below. The target may be used in a    PVD process to form a thin-film semiconductor device such as a    photovoltaic device. The raw semiconductor material is pre-reacted    to achieve the molecular structure and bonding of the compound. The    material may be amorphous, crystalline or polycrystalline in    morphology.-   2. In another embodiment, a target as described in Example 1,    comprises II-VI compound semiconductor materials.-   3. In another embodiment a target as described in Example 2    comprises Cd—S.-   4. In another embodiment a target as described in Example 2    comprises Cd—Se.-   5. In another embodiment a target as described in Example 2    comprises Cd—Te.-   6. In another embodiment a target as described in Example 2    comprises Cd—Zn—Te.-   7. In another embodiment a target as described in Example 2    comprises Cd—Hg—Te.-   8. In another embodiment a target as described in Example 1    comprises Cu—In—Se.-   9. In another embodiment a target as described in Examples 3-8 which    includes Cu as a minor constituent.-   10. In another embodiment a target as described in Example 1    comprises compound semiconductor materials.-   11. In another embodiment a target as described in Example 10    comprises Cu—In—Se. The target preferably has a polycrystalline form    of CuInSe₂.-   12. In another embodiment a target as described in Example 10    comprises Cu—In—Ga—Se. The target preferably has a polycrystalline    form of CuIn_((1-x))Ga_(x)Se₂, where x may be in the range from 0 to    1, preferably between 0.2 and 0.7, more preferably between 0.3 and    0.5. Furthermore, the target may have a preferred composition of    CuIn_(0.7)Ga_(0.3)Se₂, which may be most suitable for absorber film    deposition for use in photovoltaic devices.-   13. In another embodiment a target as described in Example 10    comprises Cu—In—Ga—Se—S. The target preferably has a polycrystalline    form of CuIn_((1-x))Ga_(x)(Se_((1-y))S_(y))₂, where x and y may be    in the range from 0 to 1. Furthermore, x and y may be preferably    between 0.2 and 0.7 and between 0 and 0.6. Furthermore, the target    may have a preferred composition of    CuIn_(0.4)Ga_(0.6)(Se_(0.4)S_(0.6))₂, which may be most suitable for    wide-bandgap absorber film deposition for use in photovoltaic    devices.-   14. In another embodiment a target as described in Examples 12 and    13 which includes Al in complete or partial substitution for Ga. The    target may have a preferred composition of CuIn_(0.5)Al_(0.5)Se₂,    which may be suitable for wide-bandgap absorber film deposition for    use in photovoltaic devices.-   15. In another embodiment a target as described in Examples 10-14    which includes Na as a minor constituent. The target preferably    contains less than 1 at. % of sodium, more preferably less than 0.1    at. % of sodium.-   16. In another embodiment a target as described in Examples 10-15    which includes alkali elements other than Na as minor constituents.-   17. In another embodiment targets as described in Examples 10-16    which includes F as a minor constituent.-   18. In another embodiment targets as described in Examples 10-17    which include a halogen element other than F as minor constituents.-   19. In another embodiment a RTPD method is used to form a    homogeneous thin-film from a compound semiconductor target which    closely replicates the composition of the target.-   20. In another embodiment a RTPD method is used to form a    homogeneous thin-film from a compound semiconductor target, in    combination with other material sources for simultaneous or    sequential depositions.-   21. In another embodiment a method as described in example 20 is    used which includes multiple RTPD target deposition steps.-   22. In another embodiment a method as described in example 20 is    used along with additional deposition steps other than RTPD    deposition steps.-   23. In another embodiment a method as described in Examples 19-22 is    used which includes magnetron sputtering.-   24. In another embodiment a method as described in Example 19-22 is    used which includes laser ablation.-   25. In another embodiment a method as described in Example 19-22 is    used which includes nano-particle deposition.-   26. In another embodiment, the RTPD method as described above may be    used to deposit a thin-film of a compound semiconductor onto a    substrate, which may be flat, textured or curved. The compound    semiconductor material may be CuInSe₂ and the resulting film may    have a thickness in the range of 1-10 microns, preferably about 1-2    microns. The substrate material may be glass, preferably soda lime    glass or Corning 1737 glass having a coefficient of thermal    expansion (CTE) close to that of CIS-type materials. The substrate    may further include additional thin-film layers, for example such as    dielectric barrier layers maid from SiO₂ or Si₃N₄, or conducting    layers maid from Mo or W films.-   27. In another embodiment, the thin-film described above may be    modified to include a thin-film of CuIn_(0.7)Ga_(0.3)Se₂.-   28. In another embodiment, the thin-film described above may be    modified to include a thin-film of CuIn_(0.5)Al_(0.5)Se₂.-   29. In another embodiment, the thin-film described above may be    modified to include a thin-film of CuInS₂.-   30. In another embodiment, the thin-film described above may be    modified to include a thin-film of CuGaSe₂.-   31. In another embodiment, the thin-film described above may be    modified to include a thin-film of CdTe.-   32. In another embodiment, the thin-film described above may be    modified to include a thin-film of a compound semiconductor having    thickness of less than 1 micron, and preferably less than 0.5    micron, which leads to cost reductions in the manufacturing of    electro-optic devices containing such a film.-   33. In another embodiment, the thin-film described above may be    modified by annealing at temperature of at least 450° C., preferably    at 500° C. or above, which would promote crystal grain growth and    improve electro-optic properties of this film, such as electron and    hole mobilities, carrier lifetime, and optical quantum efficiency.-   34. In another embodiment, the thin-film may be deposited on a    stainless steel substrate. The stainless steel may further contain    at least 13% of Cr, preferably 16-18% of Cr. The stainless steel    substrate may also be polished to have surface roughness of less    than 10 nm, and preferably of less than 2 nm.-   35. In another embodiment, the thin-film may be deposited on a    polyimide substrate. Furthermore, the film may be annealed at    temperatures of at least 400° C.-   36. In another embodiment, the thin-film may be deposited on a low    temperature polymer substrate, such as polyamide or polyethylene    terephthalate (PET). Furthermore, the film may be annealed at    temperatures of 300° C. or less. The RTPD process may enable grain    growth at lower temperatures as compared to those of regular film    deposition approaches.-   37. In yet another embodiment, the RTPD method described above may    be used to produce an electro-optic device that includes at least    one semiconductor junction having a thin-film of a compound    semiconductor deposited using the RTPD method. The junction may be    formed at the interface between said RTPD thin-film and another    semiconductor film. The conduction types may be opposite on opposite    sides of the junction so that the junction may be a PN type    junction. In addition, the device may include at least two    conducting layers and a substrate for supporting all of the    described layers. At least one the conducting layers may be    transparent.-   38. In another embodiment, the device described above is a    photovoltaic (PV) device.-   39. In another embodiment, the device described above includes a    heterojunction.-   40. In another embodiment, the device described above includes a MIS    (metal-insulator-semiconductor) type junction.-   41. In another embodiment, the device described above includes a    thin-film of a I-III-VI compound semiconductor, such as a CIGS-type    material.-   42. In another embodiment, the device described above includes a    thin-film of a II-VI compound semiconductor, such as CdTe.-   43. In another embodiment, the device described above includes a    stack of a glass substrate, Mo metal layer, CIS layer, CdS layer,    i-ZnO layer and n-ZnO layer. It may also be preferred to omit the    CdS layer and avoid Cd contamination without detrimental effects to    the device performance.-   44. In another embodiment, the device described above includes a    thin-film of wide-bandgap compound semiconductor having a bandgap    greater than 1.4 eV, preferably greater than 1.55 eV, and more    preferably greater than 1.7 eV.-   45. In another embodiment, the device described above includes a    homogeneous thin-film of wide-bandgap compound semiconductor having    an open circuit voltage greater than 0.8 V, preferably more than 0.9    V.-   46. In another embodiment, the device described above includes a    homogeneous thin-film of a compound semiconductor having a large    area of greater than 100 cm² and power conversion efficiency greater    than 15%.-   47. In another embodiment, the device described above includes a    homogeneous thin-film of CIGS-type semiconductor having a large area    of greater than 100 cm² and a power conversion efficiency greater    than 15%.-   48. In yet another embodiment, the RTPD method described above may    be used to produce an electro-optic device that includes a plurality    of modules. Each module has at least one semiconductor junction    formed by a thin-film of a compound semiconductor deposited using    the RTPD method and another adjacent semiconductor layer. Such a    device may be a multi junction PV device, which is known to be    potentially a more efficient PV device than single-junction PV    devices. A multi junction PV device requires different semiconductor    layers, each having a different bandgap. For example, in the case of    a triple junction PV device, it may be preferred to have    semiconductors with bandgaps of about 1.0 eV, 1.4 and 1.8 eV.    Standard CIGS film deposition methods fail to produce homogeneous    films with a uniform bandgap across the film; the resulting films    are often characterized by poorly defined and controlled    graded-bandgap profiles. Thus, it has been difficult to define a    bandgap in a multijunction PV device using standard approaches. On    the other hand, the RTPD method is more suitable to production of    multi junction devices, since well-defined bandgap materials can be    easily produced and precisely deposited across large areas.-   49. In another embodiment, the device described above includes    homogeneous thin-films of three different CIGS type semiconductors:    CuInSe₂, CuIn_(0.6)Ga_(0.4)Se₂ and    CuIn_(0.6)Ga_(0.4)(S_(0.6)Se_(0.4))₂.-   50. In another embodiment, the RTPD semiconductor thin-film    semiconductor materials described above may be incorporated in a    photovoltaic device of the types disclosed in copending U.S.    application Ser. No. 12/034,944 entitled “METHOD AND APPARATUS FOR    MANUFACTURING MULTI-LAYERED ELECTRO-OPTIC DEVICES,” which is hereby    incorporated by reference in its entirety-   51. In another embodiment, the RTPD semiconductor thin-film    semiconductor materials described above may be formed on a substrate    in which an electrically conducting grid is embedded, which is    disclosed in copending U.S. application Ser. No. 12/038,893 entitled    “METHOD AND APPARATUS FOR FABRICATING COMPOSITE SUBSTRATES FOR    THIN-FILM ELECTRO-OPTICAL DEVICES,” which is hereby incorporated by    reference in its entirety.-   52. In another embodiment, a target is provided which is comprised    of a II-VI compound semiconductor, with a composition formulated to    give a specific bandgap in the resultant thin-films. Specifically,    the target may comprise the elements Cd, Te, Se, and/or S.-   53. In another embodiment the target described in 52 is processed at    temperatures above the liquidus temperature of the compound    semiconductor.-   54. In another embodiment, a set of targets as described in 52 with    compositions designed to give complimentary band gaps for a multi    junction device.-   55. In another embodiment, a target as described in 52 is doped with    Cu to improve film performance.-   56. In another embodiment, a pre-reacted target is provided which is    comprised of a I-III-VI compound semiconductor, with a composition    formulated to give a specific bandgap in the resultant thin-films.    Specifically, the target may be comprised of the elements Cu, In,    Ga, Al, S, Se and/or Te.-   57. In another embodiment the target described in 55 is processed at    temperatures above the liquidus temperature of the compound    semiconductor.-   58. In another embodiment, a set of targets is provided as described    in 55 with compositions designed to give complimentary band gaps for    a multi junction device.-   59. In another embodiment, a target as described in 55 is doped with    Na or Li to improve film performance.-   60. In another embodiment, a target as described in 55 is doped with    F or Cl to improve film performance.

FIG. 2 is a flowchart showing one example of a process that may beemployed to fabricate a thin-film semiconductor device such as aphotovoltaic device. The method begins in step 110 when the absorberlayer of the semiconductor device is designed by selecting theconstituent elements or compounds. Next, in step 120 these constituentmaterials are mixed together and, in step 130, undergo a pre-reactingprocess to form a homogeneous compound semiconductor target material(i.e., an RTPD target material). The target material is optionallyreshaped or otherwise reconfigured in step 140 as appropriate for thedeposition technique that will be employed. The target material is thendeposited on a suitable substrate by the selected deposition techniquein step 150. The resulting thin-film has a composition that issubstantially the same as the composition of the compound semiconductortarget material. Finally, in step 160 any post-processing that isnecessary is performed on the thin-film.

1. A method of fabricating a light-absorbing thin-film, comprising:providing a plurality of raw semiconductor materials; pre-reacting theraw semiconductor materials to form a homogeneous compound semiconductortarget material; and depositing the compound semiconductor targetmaterial onto a substrate to form a light-absorbing thin-film having acomposition substantially the same as a composition of the compoundsemiconductor target material.
 2. The method of claim 1 wherein the rawsemiconductor materials include II-VI compound semiconductor materials.3. The method of claim 1, wherein the raw semiconductor materialsinclude I-III-VI compound semiconductor materials.
 4. The method ofclaim 2 wherein the IIB-VIA compound semiconductor materials areselected from the group consisting of Cd—S, Cd—Se, Cd—Te, Cd—Zn—Te,Cd—Hg—Te.
 5. The method of claim 3 wherein the IB-IIIA-VIA compoundsemiconductor materials are selected from the group consisting ofCu—In—Se.
 6. The method of claim 3 wherein the IB-IIIA-VIA compoundsemiconductor materials are selected from the group consisting ofCu—In—Se, with Al and, or Ga as partial or complete compositionalsubstitutes for Ga.
 7. The method of claim 3 wherein the IB-IIIA-VIAcompound semiconductor materials are selected from the group consistingof Cu—In—Se, with Ag partial replacing Cu.
 8. The method of claim 3wherein the IB-IIIA-VIA compound semiconductor materials are selectedfrom the group consisting of Cu—In—Se, with S or Te as partial orcomplete replacements for Se.
 9. A light-absorbing thin-film fabricatedin accordance with the method set forth in claim
 1. 10. The method ofclaim 1 wherein the raw semiconductor materials are pre-reacted above aliquidus temperature of the compound semiconductor target material. 11.The method of claim 3 wherein the I-III-VI compound semiconductormaterials are selected from the group consisting of Cu—In—Se,Cu—In—Ga—Se, Cu—In—Ga—Se—S.
 12. The method of claim 11 furthercomprising providing Al in complete or partial substitution for Ga. 13.The method of claim 11 further comprising providing Na as a minorconstituent along with the I-III-VI compound semiconductor materials.14. The method of claim 11 further comprising providing an alkalielement other than Na as a minor constituent long with the I-III-VIcompound semiconductor materials.
 15. The method of claim 14 furthercomprising providing F as a minor constituent along with the I-III-VIcompound semiconductor materials.
 16. The method of claim 14 furthercomprising providing a halogen element other than F as a minorconstituent along with the I-III-VI compound semiconductor materials.17. The method of claim 1 wherein the compound semiconductor targetmaterial is deposited by physical vapor deposition.
 18. The method ofclaim 17 wherein the compound semiconductor target material is depositedby magnetron sputtering.
 19. The method of claim 17 wherein the compoundsemiconductor target material is deposited by laser ablation.
 20. Themethod of claim 1 wherein pre-reacting the raw semiconductor materialsincludes isolating the raw semiconductor materials at an elevatedtemperature to establish a structure and bonding profile representativeof the deposited thin-film.
 21. A method of fabricating a photovoltaicdevice, comprising: forming a first electrode on a substrate; forming alight absorbing layer on the first electrode, wherein forming the lightabsorbing layer includes: providing a plurality of raw semiconductormaterials; pre-reacting the raw semiconductor materials to form ahomogeneous compound semiconductor target material; and depositing thecompound semiconductor target material onto a substrate to form athin-film having a composition substantially the same as a compositionof the compound semiconductor target material; and forming a secondelectrode over the light absorbing layer.
 22. A photovoltaic devicefabricated in accordance with the method set forth in claim
 21. 23. Thephotovoltaic device of claim 22 wherein the raw semiconductor materialsinclude II-VI compound semiconductor materials.
 24. The photovoltaicdevice of claim 22, wherein the raw semiconductor materials includeI-III-VI compound semiconductor materials.
 25. The photovoltaic deviceof claim 23 wherein the IIB-VIA compound semiconductor materials areselected from the group consisting of Cd—S, Cd—Se, Cd—Te, Cd—Zn—Te,Cd—Hg—Te.
 26. The photovoltaic device of claim 24 wherein theIB-IIIA-VIA compound semiconductor materials are selected from the groupconsisting of Cu—In—Se.
 27. The photovoltaic device of claim 24 whereinthe IB-IIIA-VIA compound semiconductor materials are selected from thegroup consisting of Cu—In—Se, with Al and, or Ga as partial or completecompositional substitutes for Ga.
 28. The photovoltaic device of claim24 wherein the IB-IIIA-VIA compound semiconductor materials are selectedfrom the group consisting of Cu—In—Se, with Ag partial replacing Cu. 29.The photovoltaic device of claim 24 wherein the IB-IIIA-VIA compoundsemiconductor materials are selected from the group consisting ofCu—In—Se, with S or Te as partial or complete replacements for Se. 30.The photovoltaic device of claim 22 wherein the raw semiconductormaterials are pre-reacted above a liquidus temperature of the compoundsemiconductor target material.